Some commercially available analogue electronics based single frequency sinusoidal signal transmitting metal detectors have switches which allow a user to select different frequencies. The electronics in such detectors is often expensive. The ability to select different frequencies may be useful, for example, in gold nugget prospecting where the size of gold nuggets may vary from location to location and hence the optimal frequency for detection may also vary.
U.S. Pat. No. 5,537,041 discloses a metal detector which transmits multi-period pulses and operates in the time domain, as too do most, and probably all, commercially available pulse induction metal detectors; for example those disclosed in U.S. Pat. No. 4,868,504 and U.S. Pat. No. 5,576,624. Time domain detectors are relatively highly susceptible to electromagnetic interference owing to wide receive bandwidths.
U.S. Pat. No. 4,628,265 discloses a frequency domain metal detector which applies a voltage square-wave signal to a magnetic transmitter and the received fundamental and third harmonics are band-pass filtered and then synchronously demodulated. With this approach, only two frequencies are detected simultaneously and owing to the narrow receive bandwidths of the band-pass filters, the metal detector will not be highly susceptible to electromagnetic interference.
U.S. Pat. No. 4,942,360 discloses a metal detector which detects more than one frequency simultaneously and operates in the frequency domain. This is useful to reject magnetic soil signals measure and also to characterize targets.
JEEG, volume 2, issue 1, March 1997, p 53-64 “GEM-3: A Monostatic Broadband Electromagnetic Induction Sensor” I J Won et al.
This paper discloses a geological prospecting detector that first takes a series of measurements, then multiplies the collected data by sine and cosine weighting functions for different frequencies. The outputs do not then pass through low pass filters. An output result is obtained in about 1 second, not in real time.
Some commercially available metal detectors use digital signal processing technology to use sine wave weighted synchronously demodulation. At least one of these detectors also applies a square-wave voltage to a magnetic transmitter and also detects the fundamental and third harmonic in the frequency domain as does U.S. Pat. No. 4,628,265 but achieves this by sine-wave weighted synchronously demodulation rather than exploiting the use of band-pass filters.